Triplet BODIPY and AzaBODIPY Derived Donor-acceptor Dyads: Competitive Electron Transfer versus Intersystem Crossing upon Photoexcitation

The bis-iodo beta-pyrrole-substituted BF2-chelated dipyrromethene, I2BODIPY, and its structural analogue BF2-chelated aza dipyrromethene, I(2)azaBODIPY, carrying a nitrogen at the meso-position instead of carbon, were synthesized and characterized as new set of triplet sensitizers using different techniques. These sensitizers were further functionalized with fullerene, C-60, at the central boron atom to build donor-acceptor conjugates. Using spectral, electrochemical, and computational methods, these conjugates were characterized, and the energy levels were established. Intersystem crossing to populate the triplet state was observed upon excitation of I2BODIPY and I(2)azaBODIPY, however, the measured rates of k(ISC) were found to be nearly two orders of magnitude higher for I(2)azaBODIPY (k(ISC)similar to 10(11) s(-1)) compared to I2BODIPY (k(ISC)similar to 10(9) s(-1)). The energetics, k(ISC), and position of HOMO and LUMO levels was found to control the ability of the dyad to undergo electron transfer, although the donor-acceptor distances were virtually the same in both I2BODIPY-C-60 and I(2)azaBODIPY-C-60 conjugates. Free-energy calculations revealed that the photoinduced electron transfer process was thermodynamically feasible from only the singlet excited states in both conjugates. Consequently, electron transfer from the (I2BODIPY)-I-1* in competition with intersystem crossing was witnessed in the case of I2BODIPY-C-60 dyad while in the case of I(2)azaBODIPY-C-60 dyad, excitation of azaBODIPY led to a short-lived charge transfer state involving the catechol bridge followed by populating the low-lying (3)I(2)azaBODIPY* state without promoting the process of charge separation involving C-60. The lifetime of the charge-separated states was in the ns range in the I2BODIPY-C-60 conjugate both in polar and nonpolar solvents.

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